Method and apparatus for transmission management in a wireless communication system

ABSTRACT

A method and apparatus may be used in wireless communications. The apparatus may be an access point (AP), and may transmit a power save frame. The power save frame may include one or more Uplink (UL) Transmission Times (ULT)s. The apparatus may determine that a station (STA) did not transmit during its respective ULT. The AP may transmit another power save frame. The other power save frame may include a modified ULT. The modified ULT may be for a STA that did not transmit during its respective ULT. The other power save frame may include an unmodified ULT. The unmodified ULT may be for a STA that did not transmit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/533,072 filed Sep. 19, 2006, which issued as U.S. Pat. No. 8,619,658on Dec. 31, 2013, which claims the benefit of U.S. ProvisionalApplication No. 60/719,035, filed Sep. 21, 2005, U.S. ProvisionalApplication No. 60/720,967, filed Sep. 27, 2005, and U.S. ProvisionalApplication No. 60/736,255, filed Nov. 14, 2005, the contents of whichare hereby incorporated by reference.

FIELD OF INVENTION

The present invention is related data transmission in a wirelesscommunication system. In particular, the present invention relates to amethod and apparatus for transmission management for multiple pollingand power saving in a wireless communication system.

BACKGROUND

The implementation of proposed IEEE 802.11 standards, and in particularthe IEEE 802.11n standard, will allow for higher throughput (HT)wireless local area network (WLAN) devices. One such way in which higherthroughput may be achieved is through the use of signal aggregation inboth the medium access control (MAC) layer and the physical (PHY) layer.When an aggregate is addressed to a single receiver address, it isreferred to as a Single Receiver Aggregate (SRA). When the aggregate isaddressed to multiple receivers, it is referred to as a MultipleReceiver Aggregate (MRA).

An MRA may be transmitted during a Multiple Receiver AggregateMulti-Poll (MMP) sequence or a Power Save Aggregation Descriptor (PSAD).This aggregation tends to improve system performance and also provides apower saving mechanism in the case of MMP/PSAD.

One or more MAC Service Data Units (MSDUs) being sent to the samereceiver can be aggregated into a single Aggregate-MSDU (A-MSDU). Thisaggregation of more than one frame improves the efficiency of the MAClayer, particularly when there are many small MSDUs such as TransmissionControl Protocol Acknowledgements (TCP ACKs). The overhead associatedwith channel access, such as the Physical Layer Convergence Protocol(PLCP) preamble, MAC header, and IFS spacing, can thereby be amortizedover two or more MSDUs. Additionally, a STA may only use MSDUaggregation where it knows that the receiver supports MSDU aggregation.In some cases, support for MSDU aggregation may be mandatory at thereceiver.

FIG. 1 shows an exemplary A-MSDU frame 10. The A-MSDU frame 10 includesa plurality of Sub-frame header fields 11 and a plurality of MSDU fields12 (designated MSDU₁ . . . MSDU_(n)). Each Sub-frame header field 11includes an MSDU length field 13, a source address (Source Addr) field14, and a destination address (Dest Addr) field 15. Typically, thesub-frame header fields 11 separate the MSDU to aid a receiver indeciphering whether or not the frame is directed toward it. Ordinarily,the MSDU length field 13 includes the length, the Source Addr field 14includes the address of the transmitter, and the Dest Addr field 15includes the address of the receiver. In general, to form an A-MSDU 10,two or more MSDUs are aggregated together.

Another type of aggregation may be formed by joining multiple MACProtocol Data Units (MPDUs) together. FIG. 2 shows an exemplaryaggregated MPDU (A-MPDU) frame 20. The A-MPDU frame 20 includes aplurality of MPDU delimiter fields 21 and a plurality of MPDU fields 22(designated MPDU₁ . . . MPDU_(n)). Each MPDU delimiter field 21 alsoincludes a reserved field 31, an MSDU length field 24, a CyclicRedundancy Check (CRC) field 25, and a Unique Pattern field 26. TheA-MPDU frame 20 is typically transported in a single aggregate PLCPService Data Unit (A-PSDU). Additionally, padding octets (not shown) areappended, if needed, to make each MPDU field 22 section a multiple offour octets in length, except in the case of MPDU_(n).

One purpose of the MPDU delimiter field 21 is to delimit the MPDUs 22within the aggregate. For example, the structure of the aggregate canusually be recovered when one or more MPDU delimiters are received witherrors. Also, individual MPDU delimiter fields 21 have the same blockerror rate (BER) as the surrounding MPDUs 22, and can therefore be lostduring transmission.

One advantage in using A-MPDU frames 20 is that, unlike A-MSDUs, theycan be aggregated to multiple receivers. That is, a multiple-receiveraggregate (MRA) may contain MPDUs that are addressed to multiplereceivers. Moreover, an MRA may be transmitted in one of two contextsthat are distinguished by whether it is transmitted during an MMP/PSADsequence or not. If multiple responses are required, they may bescheduled by transmission of an MMP or PSAD frame.

FIG. 3 shows a typical multiple receiver aggregate multi-poll (MMP)frame 30. The MMP frame 30 includes a frame control field 31, a durationfield 32, a receiver address (RA) field 33, a transmitter address (TA)field 34, a number of receivers (N) field 35, a receiver information(info) field 36, and a frame checksequence (FCS) field 37. The RA field33 is typically the broadcast address of a group. The TA field 34 istypically the address of the wireless transmit/receive unit (WTRU)transmitting the MRA aggregate. The number of receivers (N) field 35includes the number of receivers for which MPDUs are included in the MRAaggregate.

Additionally, the receiver info field 36 includes a plurality ofsubfields, such as an association identifier (AID) field 61, atransmission identifier (TID) field 62, a new PPDU flag field 63, areserved field 64, a receive (Rx) offset field 65, an Rx duration field66, a transmit (Tx) offset field 67, and a Tx duration field 68. The AIDfield 61 identifies a station (STA) addressed by the frame. The TIDfield 62 defines the TID for transmissions by a STA. The new PPDU flagfield 63 indicates that the downlink (DL) for the STA begins at thestart of the PPDU. The Rx offset field 65 defines the start of the firstsymbol containing DL data for a STA. The Rx duration field 66 definesthe length of a downlink. The Tx offset field 67 defines the time whentransmissions by the STA may begin, and the Tx duration field 68 definesthe duration limit of the transmissions.

FIG. 4 shows a typical power save aggregation descriptor (PSAD) frame40. The PSAD frame 40 includes a frame control field 41, a durationfield 42, an RA field 43, a TA field 44, a basic service set identifier(BSSID) field 45, a PSAD parameter (PARAM) field 46, a number ofreceivers field 47, and an FCS field 48. The PSAD PARAM field 46 furtherincludes a reserved field 71, a More PSAD indicator 72, and a descriptorend field 73. The number of receivers field 47 includes a plurality ofindividual Station Info fields which further includes a reserved field81, a STA ID field 82, a downlink transmission (DLT) start offset field83, a DLT duration field 84, an uplink transmission (ULT) start offsetfield 85, and a ULT duration field 86.

An MMP/PSAD frame may be transmitted as a non-aggregate, or may beaggregated with downlink MPDUs. Since the MMP/PSAD frame format definesreceiving and transmitting durations for each STA, it enables STAs tosave power since the STA can go into sleep mode when it is not eitherreceiving or transmitting. Also, since the MMP sequence is protectedusing a network allocation vector (NAV) and extended PHY protection(EPP), MMP provides a mechanism of scheduling multiple transmissionopportunities (TXOPs).

FIG. 5A shows an MMP/PSAD Downlink frame exchange sequence 50, and FIG.5B shows an MMP/PSAD Uplink frame exchange sequence 55. In PSAD, adownlink transmission (DLT) and an uplink transmission (ULT) period oftime are described by the PSAD frame 40. Which period of time isintended to be used for the transmission of frames from/to the PSADtransmitter to one of the PSAD receivers is also described in the PSADframe 40.

In particular, FIGS. 5A and 5B show the start offsets for DLT1 to DLTn,and ULT1 to ULTn. Similarly, in MMP, offsets are shown for a series ofdownlink transmissions RX1 to RXn and uplink transmissions TX1 to TXn.

Aggregation is also possible at the PHY-level layer for physical layer(PHY) protocol data units (PPDUs). This aggregation may be referred toas an aggregated PPDU (A-PPDU). An A-PPDU contains one or more pairs ofPLCP headers and PPDUs or PHY service data units PSDUs. To form anA-PPDU, two or more PPDUs (or PSDUs) are aggregated together, separatedby the High Throughput Signal (HT-SIG) field.

FIG. 6 shows a typical aggregated PPDU (A-PPDU) 60. The A-PPDU 60includes a legacy preamble (L-Preamble) 91, a High-Throughput Preamble(HT-Preamble) 92, a plurality of PSDU fields 93 (PSDU₁ . . . PSDU_(n)),and a plurality of HT-Signal (HT-SIG) fields 94 (HT-SIG₁ . . .HT-SIG_(n)). An HT-SIG field 94 may also include a length field 95, anMCS field 96, an advanced coding field 97, a sounding packet 98, anumber HT-Legacy Training Field (HT-LTF) 99, a Short GI field 101, a20/40 field 102, a cyclic redundancy check (CRC) field 103, and a tailfield 104.

As shown in FIG. 6, the resulting A-PPDU 60 is therefore the combinationof all PPDUs (or PSDUs) in the A-PPDU along with HT-SIGs 94 for eachconstituent PSDU 93. Since each PSDU 93 shown in FIG. 6 is delimited byan HT-SIG 94 that defines the various physical layers parameters, theA-PPDU comprises multi-rate PSDUs.

One of the drawbacks to the current system, however, is that when anMMP/PSAD is transmitted by the AP, it is possible that one or more ofthe STAs associated with the MMP/PSAD will not correctly receive, orincorrectly decode the MMP/PSAD frame. In these cases, the STAs that donot correctly receive or decode the MMP/PSAD frame will miss theirscheduled uplink transmission times, effectively wasting the WLAN mediumtime.

It would therefore be advantageous if a method and apparatus existedthat served as a mechanism to recover the structure of the A-PPDU 90 ifone or more HT-SIGs 94 or PSDUs 93 are received in error due to poorchannel conditions. It would further be advantageous for a method andapparatus to exist wherein an AP recovers any unused ULT, can transmitmultiple MMP/PSAD frames, and can schedule multicast and broadcasttransmissions in MMP/PSAD frames.

SUMMARY

In a wireless communication system comprising at least one access point(AP) and a plurality of stations (STAs), a method for transmissionmanagement of the wireless medium comprises the AP configuring aMultiple Receiver Aggregate Multi-Poll/Power Save Aggregation Descriptor(MMP/PSAD) frame with scheduled Uplink Transmission Time (ULT)information for the plurality of STAs. The AP then transmits theMMP/PSAD frame to the plurality of STAs. Upon successfully receiving anddecoding the MMP/PSAD frame, STAs transmit during their scheduled ULT.

A method and apparatus may be used in wireless communications. Theapparatus may be an access point (AP), and may transmit a power saveframe. The power save frame may include one or more Uplink (UL)Transmission Times (ULT)s. The apparatus may determine that a station(STA) did not transmit during its respective ULT. The AP may transmitanother power save frame. The other power save frame may include amodified ULT. The modified ULT may be for a STA that did not transmitduring its respective ULT. The other power save frame may include anunmodified ULT. The unmodified ULT may be for a STA that did nottransmit.

A STA may receive a power save frame. The power save frame may includeone or more ULTs. One of the ULTs may be a scheduled ULT for the STA. Ifthe STA does not transmit during the scheduled ULT, the STA may receiveanother power save frame. The other power save frame may include amodified ULT for the STA. The STA may transmit packet data based on themodified ULT. The other power save frame may include an unmodified ULTfor another STA.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe preferred embodiments of the present invention will be betterunderstood when read with reference to the appended drawings, wherein:

FIG. 1 shows an exemplary A-MSDU frame;

FIG. 2 shows an exemplary aggregated MPDU (A-MPDU) frame;

FIG. 3 shows a typical multiple receiver aggregate multi-poll (MMP)frame;

FIG. 4 shows a typical power save aggregation descriptor (PSAD) frame;

FIG. 5A shows an MMP/PSAD Downlink frame exchange sequence;

FIG. 5B shows an MMP/PSAD Uplink frame exchange sequence;

FIG. 6 shows a typical aggregated PPDU (A-PPDU);

FIG. 7 shows a wireless communication system configured in accordancewith the present invention;

FIG. 8 is a functional block diagram of an AP and a STA configured toperform a method for transmission management, in accordance with thepresent invention;

FIG. 9 is a flow diagram for managing transmission times in the wirelesscommunication system of FIG. 7, in accordance with an embodiment of thepresent invention;

FIG. 10 is an exemplary signal diagram of a downlink and uplink exchangefor the wireless communication system of FIG. 7, in accordance with anembodiment of the present invention;

FIG. 11 is an exemplary signal diagram of a downlink and uplink exchangefor the wireless communication system 100 where a particular STA did notsuccessfully receive and decode its downlink and uplink schedulinginformation;

FIG. 12 is a flow diagram of a method of recovering the medium, inaccordance with an embodiment of the present invention;

FIG. 13 is an exemplary signal diagram of a downlink and uplink exchangefor the wireless communication system, showing a broadcast or multicastMMP/PSAD transmitted during a broadcast phase of the downlink phase; and

FIG. 14 is an exemplary signal diagram of a downlink and uplink exchangefor the wireless communication system, showing a broadcast or multicastMMP/PSAD transmitted between the downlink and uplink phases.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, a station (STA) includes but is not limited to a wirelesstransmit/receive unit (WTRU), user equipment (UE), mobile station, fixedor mobile subscriber unit, pager, or any other type of device capable ofoperating in a wireless environment. When referred to hereafter, anaccess point (AP) includes but is not limited to a base station, Node-B,site controller, access point or any other type of interfacing device ina wireless environment.

FIG. 7 shows a wireless communication system 100 configured inaccordance with the present invention. The wireless communication system100 in a preferred embodiment may be a wireless local area network(WLAN), and includes an AP 110 and a plurality of STAs 120 (designatedSTA1, STA2, and STA3) capable of wireless communication with the AP 110.The AP 110, in a preferred embodiment, is connected to a network 130,such as the Internet, a public switched telephone network (PSTN), or thelike. In this manner, the STAs 120 are provided access to the network130 through the AP 110. Although only three STAs 120 are depicted in thewireless communication system 100, it should be noted that any number ofSTAs 120 may exist in the wireless communication system 100 and be incommunication with the AP 110.

FIG. 8 is a functional block diagram of an AP 110 in communication witha STA 120, configured to perform a method for transmission management inthe wireless system 100.

In addition to the components normally included in a typical AP, the AP110 includes a processor 115 configured to manage transmission in thewireless communication network 100, a receiver 116 in communication withthe processor 115, a transmitter 117 in communication with the processor115, and an antenna 118 in communication with the receiver 116 and thetransmitter 117 in order to facilitate wireless transmission andreception. Additionally, in a preferred embodiment, the processor 115 iscapable of communicating with the network 130.

In addition to the components normally included in a typical STA, theSTA 120 includes a processor 125 configured to manage transmission inthe wireless communication system 100, a receiver 126 in communicationwith the processor 125, a transmitter 127 in communication with theprocessor 125, and an antenna 128 in communication with the receiver 126and the transmitter 127 in order to facilitate wireless transmission andreception.

FIG. 9 is a flow diagram 900 for managing transmission times in thewireless communication system 100, in accordance with an embodiment ofthe present invention. In step 910, the AP 110 configures an MMP/PSADframe in order to signal to STAs 120 their transmit times in the ULphase. Specifically, the MMP/PSAD frame schedules both downlink anduplink frame exchanges for a subsequent duration of time that isspecified in the MMP/PSAD duration field. For example, the AP 110 mayachieve this by arranging the order of PSAD descriptor fields (or theMMP Receiver Information fields) according to increasing values of theRx/DLT Start Offset, or according to the order of transmission when theRx/DLT Start Offset is equal. This is particularly useful if the AP 110is sending an A-PPDU containing multiple PPDUs destined to multiplereceiver STAs 120. Additionally, the AP 110 populates the TA field(34,44) with its own identifier, such as its MAC address, and the RAfield with addresses of intended receivers. In one embodiment, the RAfield may be populated with the MAC addresses of the STAs 120 theMMP/PSAD frame is intended for.

The AP 110 then transmits the MMP/PSAD frame to the STAs 120 (step 920).Each particular STA 120 then receives the MMP/PSAD frame (step 930). Ifthe particular STA 120 receives and decodes the MMP/PSAD framesuccessfully (step 940), the STA 120 extracts it transmit time from theMMP/PSAD frame (step 950). If the STA 120 does not successfully receiveand decode the MMP/PSAD frame (step 940), then the AP 110 recovers themedium (step 970), which will be described in more detail below.

In one example, the STA 120 extracts timing information of theindividual PPDUs that form the A-PPDU aggregate. A STA 120 that receivesan MMP/PSAD frame can derive its HT-SIG time information from the Offsetfield and Duration field that are defined within the MMP/PSAD frame.Specifically, the Rx (or DLT) Start Offset and Duration fields are usedfor the purpose of extracting the HT-SIG timing information of anA-PPDU, thereby improving the reliability of the A-PPDU aggregationscheme. This may also allow a simple receiver implementation.

For purposes of example, it may be assumed that one of the aggregateswithin the MMP/PSAD exchange is an A-PPDU aggregate. For a STA 120identified in the MMP/PSAD frame as having downlink data within theMMP/PSAD exchange, the immediately preceding station's MMP/PSAD RxOffset and Rx Duration fields may be used in order to determine thestarting time of its HT-SIG field. However, this sharing of Rx Offsetinformation occurs only if both stations have the same Rx Offset.Otherwise, the Rx Offset of the particular STA 120 is used. Accordingly,by adding the Rx Offset and Rx Duration of the prior station, theparticular STA 120 can determine when its PPDU HT-SIG will start.

Alternatively, the particular STA 120 can use multiple prior fields ofthe MMP/PSAD frame instead of only one prior field, such as only theinformation of the immediately prior station. This variation may beuseful, for example, when the Rx Duration fields are not definedrelative to a common Rx Offset, but rather in terms of the actualduration of the data PPDU. In this case, the particular STA 120 may needto perform an overall addition on the all previous Rx Duration fields.

In another alternative, a field or bit is added within the MMP/PSADdescriptor fields or the MMP Receiver Information fields. This field orbit differentiates whether the timing information is related to thestart of an MPDU within an A-MPDU aggregate, or a PPDU within an A-PPDUaggregate. For example, this added field could be used to indicate thatthe particular STA 120 should expect to receive and decode an HT-SIG atthis Rx Offset, a preamble training field, or an MPDU delimiter field.

If the A-PPDU is transmitted without the MMP/PSAD, it should beprotected with a network allocation vector (NAV) setting or spoofingsince the irregular error probability and error propagation can disruptthe power savings, medium access and NAV of other STAs 120 in thesystem. For example, an A-PPDU from the AP 110 can be preceded by aclear to send (CTS)-to-self transmission to provide NAV and/or EPPprotection. An A-PPDU from a Non-AP STA 120 may be protected by anRTS/CTS exchange for NAV and EPP protection.

Once the STA 120 extracts its timing information (step 950), it thentransmits during its transmit time in the UL phase (step 960).

FIG. 10 is an exemplary signal diagram 101 of a downlink and uplinkexchange for the wireless communication system 100, in accordance withthe method 900 described above. The AP 110 transmits the MMP/PSAD framethat includes the downlink and uplink scheduling information for STA1,STA2, and STA3. In the downlink phase, the AP 110 transmits the downlinkinformation for STA1, STA2, and STA3 as indicated by D1, D2, and D3. Ifeach STA 120 successfully received and decoded the MMP/PSAD frame, theneach STA 120 (STA1, STA2, and STA3) receives its downlink informationduring its scheduled time as D1, D2, and D3, respectively. In the uplinkphase, STA1 transmits during its scheduled uplink time (U1), STA2transmits during its scheduled uplink time (U2), and STA3 transmitsduring its scheduled uplink time (U3). In this manner, each STA 120knows when it needs to be active in order to receive downlink dataassociated with it or to transmit during its scheduled uplink time.Accordingly, each STA 120 can power down during times that it knows itis not scheduled to transmit or receive, thereby allowing it to conserveits energy.

Since the scheduling of uplink and downlink frame exchanges is scheduledin the MMP/PSAD frame, a STA 120 that does not successfully receive anddecode the MMP/PSAD frame (step 940) will not be aware of its timing andmay miss its transmission opportunity in the UL. Without any mechanismto prevent or recover this from happening, the medium time may bewasted. To prevent this from occurring, the AP 110 should recover themedium (step 970).

FIG. 11 is an exemplary signal diagram 101′ of a downlink and uplinkexchange for the wireless communication system 100 where a particularSTA 120 (in this case STA2) did not successfully receive and decode itsdownlink and uplink scheduling information in step 940. Accordingly,STA2 does not transmit during its scheduled uplink time (U2′).

FIG. 12 is a flow diagram of a method of recovering the medium 970, inaccordance with an embodiment of the present invention. In step 980, theAP 110 monitors the medium to detect whether or not particular STAs 120are transmitting during their scheduled UL time. The AP 110 may utilizethe timing information in the MMP/PSAD frame to determine when it shouldmonitor the medium (step 980), or it may continuously monitor themedium.

If the AP 110 detects that a STA 120 is not conducting its uplinktransmissions when scheduled, the AP 110 may reclaim the medium (step990).

Referring again to FIG. 11, the AP 110 will monitor the Uplink Phase anddetect that STA1 transmits its uplink data during its scheduled uplinkwindow (U1). The AP 110 will then detect that STA2, for example, is nottransmitting during its scheduled uplink transmit window (U2′). Afterwaiting an idle period, AP 110 will reclaim the medium (step 990).

In a preferred embodiment, the idle period is a pre-determined periodthat the AP 110 will wait to give a STA 120 an ample opportunity tobegin transmitting during its uplink time, before the AP 110 reclaimsthe medium. As an example, the idle time period may be equal to thepoint control function inter-frame spacing (PIFS) period. The AP 110 maymonitor the medium (step 980) during the MMP/PSAD exchange period if theAP 110 is not transmitting, or since the AP 110 may know the timeperiods in which each STA 120 is to be transmitting in the uplink, theAP 110 may only monitor the medium during those times.

Alternatively, the AP 110 may monitor the medium for frame errors orcollisions that are occurring during the uplink phase, and base adecision as to reclaiming the medium on those observations.Additionally, the AP 110 may decide to cancel the MMP/PSAD in order totransmit or schedule data traffic that has a higher priority than whatthe AP 110 has already accounted for. For example, the AP 110 may wishto improve Quality of Service (QoS) requirements for particular traffic,or to schedule control traffic.

At any rate, once the AP 110 has decided to reclaim the medium in step990, there are several ways by which it may do so.

One way in which the AP 110 may reclaim the medium is by reschedulingDLT or ULT transmissions (step 991). In a preferred embodiment, the AP110 accomplishes this by transmitting a frame to indicate to all orselected STAs 120 that they should disregard the previously sentMMP/PSAD frame (step 992). This frame may have a number of formats.

For example, the frame transmitted in step 992 may be a newly definedframe to reset or cancel the prior MMP schedule, or any control,management or data frame that can be configured to indicate to the STAs120 to reset prior MMP/PSAD schedules.

In one preferred embodiment, however, the AP 110 retransmits anotherMMP/PSAD frame. The MMP/PSAD frame may be the original MMP/PSAD frame,but containing a field that specifies that the previous schedulinginformation should be ignored by all, or selected, STAs 120.Alternatively, the MMP/PSAD frame may be identical to the previouslysent MMP/PSAD frame, but with a defined rule specifying that if a STA120 receives an MMP/PSAD frame, it is to disregard any schedulinginformation received from any prior MMP/PSAD frame.

A NAV duration of the new MMP/PSAD, or any frame used to cancel or resetthe prior MMP/PSAD schedule, may be utilized to reset or update the NAVat the receiving STAs 120. Another frame, such as a CF-END frame couldalso be used to reset the NAV durations of the STAs 120. Alternatively,the wireless communication system 100 may be configured such that theduration of the most recent MMP/PSAD frame supersedes any locally storedNAV durations at the STAs 120.

Another way in which the AP 110 may reclaim the medium is bytransmitting a poll frame to the STA 120 that is not transmitting duringits scheduled transmit time (step 993). The poll frame may include acontention free poll (CF-Poll), a QoS Poll, another MMP/PSAD frame, orthe like. Alternatively, the AP 110 may transmit the poll frame to adifferent STA 120 than the scheduled STA for uplink transmission. TheSTA 120 that receives the poll frame will then begin transmitting inresponse to the poll frame (step 994). If the STA 120 has data totransmit, then it will transmit the data. Otherwise it will transmit anacknowledgement frame, a QoS Null, a Data Null, or another frame toindicate that it does not have any data to transmit.

Yet another way the AP 110 may reclaim the medium is by transmittingdownlink data and, in a preferred embodiment, a reverse direction grant(RDG) signal, as in step 995. For example, the AP 110 may send downlinkdata to any STA 120 that it desires, or the AP 110 may transmit anycontrol or management frame during this period. Upon receiving thedownlink data and RDG signal, the receiving STA transmits its uplinkdata for its time duration (step 996). Even if the AP 110 does notpossess any downlink data to transmit, it may still transmit a DataNull, QoS Null, or the like, to indicate to the non-transmitting STAthat it should begin transmitting for its specified duration.

For example, referring back to FIG. 11, if the AP 110 detects that themedium is idle for too long after STA1's uplink transmission (U1) time,then AP 110 transmits downlink data and an RDG to STA2. Upon receivingthe downlink data and RDG, STA2 transmits its data for its specifiedduration (U2′).

If the STA 120 does not have data to transmit in the uplink, the STA 120should transmit a response frame to the AP 110 such as a QoS Null frame,a Data Null frame, or the like, to indicate to the AP 110 that the STAdoes not have data to transmit during its allotted uplink time. The AP110 can thereby reclaim the medium and take some other remedial actionto avoid wasting the medium, such as polling another STA 120 to beginits transmission.

Another way that the AP 110 may reclaim the medium is by transmitting aredundant MMP/PSAD frame (step 997). The redundant MMP/PSAD frame mayrepeat some or all of the ULT information during the time when a STA 120misses its transmission window in the uplink phase. This is particularlyuseful if more than one STA 120 did not receive or decode the MMP/PSADframe successfully. The AP 110 may also decide to utilize a redundantMMP/PSAD frame if it detects certain events occurring in the wirelesscommunication system 100 during a previous MMP/PSAD exchange sequence,or because the AP 110 possesses particular knowledge about the ULTinformation or number of STAs 120, that would make it appropriate totransmit a redundant MMP/PSAD.

For example, the AP 110 may have detected in a previous MMP/PSAD frameexchange that certain STAs 120 did not transmit their information duringtheir scheduled uplink times. In this case, the AP 110 may determinethat in the next MMP/PSAD exchange, it will transmit a redundantMMP/PSAD frame to enhance the probability that all STAs 120 willproperly receive their scheduled ULTs.

Additionally, the AP 110 may know that there are a large number of STAs120 in the wireless communication system 100, and therefore, theprobability of any particular STA 120 failing to receive its ULTinformation in the first MMP/PSAD is increased. Similarly, the AP 110may have knowledge relating to extensive ULTs scheduled for the STAs 120in the wireless communication system 100, meaning that if one STA 120failed to receive the first MMP/PSAD, a large amount of wasted bandwidthcan occur if that STA fails to transmit during its scheduled ULT. Inthese scenarios, transmitting a redundant MMP/PSAD frame enhances theprobability that all the STAs 120 in the system will utilize theirscheduled ULTs, eliminating wasted bandwidth. Essentially, the AP 110may utilize a comparison of the duration of individual ULTs, the totalduration of all ULTs, and the number of STAs 120 having ULTs againstpre-determined thresholds to decide whether or not a redundant MMP/PSADframe should be transmitted.

Referring again to FIG. 11, suppose that not only STA2 failed tosuccessfully receive and decode the MMP/PSAD frame, but STA3 failed aswell. In this case, both STA2 and STA3 would miss their scheduledtransmission times without remedial action by the AP 110. Therefore, ifthe AP 110 were to detect an idle period for too long after the uplinktransmission (U1) of STA1, then the AP 110 transmits a redundantMMP/PSAD frame. In this way, STA3 receives the redundant MMP/PSAD frameand transmits its data during its scheduled uplink window (U3), thuslimiting further waste of the medium.

In yet another alternative embodiment of the present invention, the AP110 may schedule a broadcast or multicast frame utilizing the MMP/PSADframe. In order to do this, the AP 110 must reconfigure the existingPSAD frame 40 of the MMP/PSAD frame, since the current format specifiesthat the STA ID field 82 is the Association ID of the STA 120.Therefore, to support transmitting a broadcast or multicast frame withinthe MMP/PSAD sequence, the existing PSAD frame 40 should bereconfigured.

One way to reconfigure the PSAD frame 40 is to include a bit or a fieldwithin the MMP/PSAD frame. In a preferred embodiment, this is includedin the Station Info field. For example, a bit may be included in theReserved field 81 of the Station Info field, specifying that a broadcastframe will be transmitted at specified DLT parameters, and that STAsshould remain awake during that duration. Alternatively, specific valuesfor the STA ID field 82, for example all “1's”, may be utilized toindicate that a broadcast frame will be transmitted and that STAs shouldremain awake during the duration. That is, the STA ID field 82 shouldhave all bits set to “1”.

FIG. 13 is an exemplary signal diagram 131 of a downlink and uplinkexchange for the wireless communication system 100, where a broadcast ormulticast MMP/PSAD is transmitted during a broadcast phase of thedownlink phase. In the present example, the AP 110 transmits the firstMMP/PSAD prior to the downlink phase that indicates to some or all STAs120 that they should listen during a broadcast interval that will occurat the end of the downlink phase. At the end of the downlink phase, theAP 110 may then transmit a frame, and preferably an additional MMP/PSADframe to confirm the ULT schedules. In this manner, the STAs 120 willhave received their ULT schedules twice and therefore be less likely tomiss their uplink transmission time.

Alternatively, the AP 110 may insert a second MMP/PSAD frame with thefirst MMP/PSAD frame exchange sequence by including a unicast MMP/PSADentry in the first MMP/PSAD frame that describes the Tx Start Offset andthe Tx Duration for when the second MMP/PSAD frame is to be transmitted.As an example, the entry may include any MAC address of any STA 120 as adummy receiver address. This entry may also include inaccurate Tx StartOffset and Tx Duration information. At the end of the downlink phase,the AP 110 may then transmit a frame, and preferably an additionalMMP/PSAD frame to confirm the ULT schedules. In this manner, only STAs120 that have not successfully received and decoded the first MMP/PSADwill remain awake, or wake up, to receive and decode the second MMP/PSADframe, while those stations that successfully received and decoded thefirst MMP/PSAD frame will not need to wake up to receive and decode thesecond MMP/PSAD frame. This is because those STAs 120 that successfullyreceive and decode the first MMP/PSAD frame will know that the secondMMP/PSAD frame was not meant for them. Alternatively, no informationrelating to the second MMP/PSAD frame may be transmitted in the firstMMP/PSAD frame.

FIG. 14 is an exemplary signal diagram 141 of a downlink and uplinkexchange for the wireless communication system 100, where a broadcast ormulticast MMP/PSAD is transmitted between the downlink phase and theuplink phase. In this example, the AP 110 inserts a second MMP/PSADframe within the first MMP/PSAD frame, but does not include any entry inthe first MMP/PSAD frame to describe the second one. The AP 110 thentransmits the second MMP/PSAD frame to confirm the ULT schedules in aninterval between the downlink phase and the uplink phase, as shown inFIG. 14. In this scenario, only STAs 120 that did not successfullyreceive and decode the first MMP/PSAD frame will awaken to receive thesecond MMP/PSAD frame since those that did successfully receive anddecode the first MMP/PSAD frame will not awaken until they need to basedon their successful decoding of the first MMP/PSAD frame.

Importantly, however, it should be noted that the AP 110 should accountfor the effect of the inserted, or nested, MMP/PSAD frame during its ULTOffset and Duration calculations. Otherwise, the AP 110 will be out ofsynchronization with the Offsets that the STAs 120 believe they arerequired to adhere to.

This nested, or redundant MMP/PSAD frame, may or may not containidentical information to the first MMP/PSAD frame. In a preferredembodiment, however, it will contain the ULT information for the STAs120, and will typically contain consistent information with that of thefirst MMP/PSAD frame. That is, the second MMP/PSAD frame should containthe same scheduling information that was contained in the first MMP/PSADframe.

Although in previous embodiments, the AP 110 is described as monitoringthe medium in order to determine whether or not to reclaim it, STAs 120may also monitor the medium in order to further improve systemperformance. Typically, the STAs 120 that receive their ULT scheduleinformation in the MMP/PSAD frame do not perform sensing of the medium.They simply blindly begin their transmissions at their scheduled ULT.However, in some instances, it may be desirable to have the STAs 120monitor the medium instead of, or in addition to, the AP 110. In oneembodiment, a STA 120 may monitor the medium for any idle periods. Ifthe STA 120 detects an idle period lasting beyond a pre-determinedthreshold, that STA may then transmit its uplink transmission during theremaining ULT duration, thereby avoiding collisions with other STAsULTs, while maximizing use of the medium.

Although FIG. 1 only depicts one AP 110, it is also possible for severalAPs to be present in a wireless communication system. In this case, someSTAs in the wireless communication system may be associated with one AP,while other STAs may be associated with other APs, which could causesome difficulty. In one scenario, one of the APs (AP1) may be associatedwith an Overlapping Basic Service Set (OBSS) or a co-channel BSS toanother AP (AP2). If AP1 were to transmit an MMP/PSAD frame, STAsassociated with AP2 may ignore the MMP/PSAD frame after receiving itbecause they will not see in the RA field any address that wouldindicate to them that the frame is intended for them, and go into asleep state. If AP2 then transmits traffic during that time, theintended STAs will not receive the information because they will havebeen asleep during the transmission.

Accordingly, the STAs 120 receiving an MMP/PSAD frame may be configuredto read the TA field in the frame to determine if the MMP/PSAD frame wassent from an AP with which the STA is associated. If the STA determinesthat the AP address in the TA field is the address of the AP the STA isassociated with, then the STA can decode downlink transmissions andperform uplink transmissions in accordance with the contents of theMMP/PSAD frame, while going into sleep mode at other times. Conversely,if the STA determines that the AP transmitting the MMP/PSAD frame is notthe AP the STA is associated with, it may ignore the frame, but stillremain in an awake state to receive any transmission that might be sentfrom its associated AP. Additionally, however, the STA may desire toread the Duration ID value in an MMP/PSAD frame and update its NAVDuration, even if the frame was not sent by an associated AP. In thismanner, the STA will know when the medium will be in use, and will beable to avoid transmitting during those times.

In another alternative embodiment of the present invention, the MMP/PSADframe may be utilized to poll certain types of packets, such as BlockAcknowledgement (BA) response frames. In this case, the AP 110 mayutilize one or more flags within the MMP/PSAD frame indicating tospecified STAs 120 that they transmit their BA response frames duringtheir scheduled ULTs. The flag may further indicate to the STAs 120whether or not they are to transmit only BA response frames during theirscheduled ULT, or if they are to transmit BA response frames along withother frames the STA is transmitting. This alternative facilitates theaddition of a new mode for BA to the currently existing modes.

Currently, the existing BA modes include Immediate Block ACK and DelayedBlock ACK. In Immediate Block ACK mode, a STA responds to a BA request(BAR) immediately following an SIFS delay. In Delayed Block ACK mode, aSTA decides on its own when to transmit a BA frame.

The present alternative embodiment includes polling for a Delayed BA, asopposed to the BA being transmitted at an arbitrary time by the STA. Forexample, the AP 110 may transmit a BAR to the STA 120 indicating to theSTA 120 that the STA 120 should prepare the BA packet, or BA frame, andonly transmit the packet when the STA 120 receives another poll messagefrom the AP 110. The poll message may be in the form of an MMP oranother packet transmitted by the AP 110. This provides for the AP 110to determine the time for associated STAs 120 to transmit their BAframes instead of having the STAs themselves determine when to transmitthem.

The above features may be implemented in a wireless transmit/receiveunit (WTRU), base station, and/or peer-to-peer devices. The abovemethods are applicable to a physical layer and/or a data link layer. Theapplicable forms of implementation include application specificintegrated circuit (ASIC), middleware, and software. This invention canbe applied in an OFDM/MIMO system and a IEEE 802.11 compliant system.

Additionally, the features of the embodiments of the present inventionmay be implemented in a variety of manners, such as in an applicationrunning on a WTRU, such as an AP or STA. The features may also beincorporated into an integrated circuit (IC) or be configured in acircuit comprising a multitude of interconnecting components.Additionally, the features may be performed by a software applicationthat runs on an IC, or by a software application that runs on aprocessor.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone (without the other features andelements of the preferred embodiments) or in various combinations withor without other features and elements of the present invention.

What is claimed is:
 1. A method for use in an access point (AP), themethod comprising: transmitting, to a plurality of stations (STA)s, afirst power save frame that includes a plurality of scheduled Uplink(UL) Transmission Times (ULT)s; determining that one of the plurality ofSTAs did not transmit during its respective ULT; and transmitting asecond power save frame to the plurality of STAs, wherein the secondpower save frame includes a modified ULT for the STA that did nottransmit during its respective ULT and an unmodified ULT for a STA witha ULT scheduled after the STA that did not transmit.
 2. The method ofclaim 1, further comprising: waiting a predetermined period beforetransmitting the second power save frame.
 3. The method of claim 2,wherein the predetermined period is a point control function inter-framespacing (PIFS) period.
 4. The method of claim 1, wherein the secondpower save frame includes rescheduled ULTs.
 5. The method of claim 4,further comprising: transmitting an additional frame to at least one STAindicating that the at least one STA should ignore a previouslytransmitted power save frame.
 6. The method of claim 5, wherein theadditional frame is transmitted to all of the plurality of STAs.
 7. Themethod of claim 5, wherein the additional frame is transmitted to aportion of the plurality of STAs.
 8. The method of claim 5, wherein theadditional frame is a control frame, a data frame, a management frame,or a Multiple Receiver Aggregate Multi-Poll/Power Save AggregationDescriptor (MMP/PSAD) (MMP/PSAD) frame.
 9. The method of claim 8 whereinthe additional frame is a third power save frame including theindication to ignore the first power save frame.
 10. The method of claim9 wherein the additional frame is a third power save frame, and whereinthe third power save frame is identical to the first power save frame.11. The method of claim 5, further comprising: transmitting, on acondition that the AP fails to receive an uplink transmission from oneof the plurality of STAs during a respective ULT, a contention free end(CF-END) frame to the STA to update a network allocation vector (NAV)duration at the STA.
 12. The method of claim 1 wherein the second powersave frame is a poll frame.
 13. The method of claim 12, wherein the pollframe includes any one of a contention free poll (CF-Poll) or a qualityof service (QoS) poll.
 14. The method of claim 12, further comprising:receiving a response from one of the plurality of STAs in response tothe poll frame.
 15. The method of claim 14, wherein data is received inresponse to the poll frame.
 16. The method of claim 14, wherein anacknowledgement frame, a QoS Null frame, or a Data Null frame, isreceived in response to the poll frame.
 17. The method of claim 1,further comprising: transmitting downlink data and a reverse directiongrant (RDG) signal to at least one STA.
 18. The method of claim 17,wherein downlink data is transmitted to the STA that is not transmittingduring the scheduled ULT.
 19. The method of claim 17, furthercomprising: polling another STA to begin transmitting on a conditionthat a Data Null frame or a QoS Null frame is received in response tothe RDG.
 20. The method of claim 1, wherein the second power save frameis a redundant Multiple Receiver Aggregate Multi-Poll/Power SaveAggregation Descriptor (MMP/PSAD) frame.
 21. The method of claim 20,wherein the redundant MMP/PSAD frame includes all of the ULT informationincluded in a previous MMP/PSAD frame.
 22. The method of claim 20,wherein the redundant MMP/PSAD frame is transmitted during a downlinkphase.
 23. The method of claim 20, wherein the redundant MMP/PSAD frameis transmitted between a downlink phase and an uplink phase.
 24. Themethod of claim 1, wherein transmitting the second power save frame isin response to detecting a frame error during an uplink phase.
 25. Themethod of claim 1, wherein transmitting the second power save frame isin response to detecting an uplink transmission collision during anuplink phase.
 26. The method of claim 1, further comprising transmittinga redundant Multiple Receiver Aggregate Multi-Poll/Power SaveAggregation Descriptor (MMP/PSAD) frame based upon a prior knowledge ofconditions in a wireless communication system.
 27. The method of claim1, wherein the first power save frame includes a field that indicates abroadcast transmission.
 28. The method of claim 27, wherein the fieldincludes a broadcast transmission parameter.
 29. The method of claim 1,wherein the first power save frame is a Multiple Receiver AggregateMulti-Poll/Power Save Aggregation Descriptor (MMP/PSAD) frame.
 30. Themethod of claim 1, wherein the second power save frame is a MultipleReceiver Aggregate Multi-Poll/Power Save Aggregation Descriptor(MMP/PSAD) frame.
 31. A station (STA) comprising: a transmitter; areceiver configured to receive a first power save frame that includes aplurality of Uplink (UL) Transmission Times (ULT)s, wherein one of theplurality of ULTs is a scheduled ULT for the STA; wherein, on acondition that the transmitter does not transmit during the scheduledULT for the STA, the receiver is further configured to receive a secondpower save frame that includes a modified ULT for the STA and anunmodified ULT for a STA with a ULT scheduled after the STA; and whereinthe transmitter is configured to transmit packet data based on themodified ULT.
 32. The STA of claim 31, wherein the first power saveframe is a Multiple Receiver Aggregate Multi-Poll/Power Save AggregationDescriptor (MMP/PSAD) frame.
 33. The STA of claim 31, wherein the secondpower save frame is a Multiple Receiver Aggregate Multi-Poll/Power SaveAggregation Descriptor (MMP/PSAD) frame.
 34. An access point (AP)comprising: a receiver; a transmitter configured to transmit, to aplurality of stations (STA)s, a first power save frame that includes aplurality of Uplink (UL) Transmission Times (ULT)s; and a processor incommunication with the receiver and the transmitter, the processorconfigured to determine that one of the plurality of STAs did nottransmit during its respective ULT, wherein the transmitter is furtherconfigured to transmit a second power save frame that includes amodified ULT for the STA that did not transmit during its respective ULTand an unmodified ULT for a STA with a ULT scheduled after the STA thatdid not transmit.
 35. The AP of claim 34 wherein the receiver is furtherconfigured to receive a request to transmit a power save frame thatincludes scheduled ULT information.
 36. The AP of claim 34 wherein theprocessor is further configured to monitor a wireless medium for idleperiods that exceed a pre-defined threshold.
 37. The AP of claim 36wherein the transmitter is further configured to transmit downlink dataon a condition that the processor detects an idle period that exceedsthe pre-defined threshold.
 38. The AP of claim 34, wherein the firstpower save frame is a Multiple Receiver Aggregate Multi-Poll/Power SaveAggregation Descriptor (MMP/PSAD) frame.
 39. The AP of claim 34, whereinthe second power save frame is a Multiple Receiver AggregateMulti-Poll/Power Save Aggregation Descriptor (MMP/PSAD) frame.
 40. Amethod for use in a station (STA), the method comprising: receiving afirst power save frame that includes a plurality of scheduled Uplink(UL) Transmission Times (ULT)s, wherein one of the plurality of ULTs isa scheduled ULT for the STA; receiving, on a condition that the STA doesnot transmit during the scheduled ULT for the STA, a second power saveframe that includes a modified ULT for the STA and an unmodified ULT fora STA with a ULT scheduled after the STA; and transmitting packet databased on the modified ULT.
 41. The method of claim 40, wherein the firstpower save frame is a Multiple Receiver Aggregate Multi-Poll/Power SaveAggregation Descriptor (MMP/PSAD) frame.
 42. The method of claim 40,wherein the second power save frame is a Multiple Receiver AggregateMulti-Poll/Power Save Aggregation Descriptor (MMP/PSAD) frame.